1. Field of the Invention
The present invention relates to a semiconductor device having thin film transistors (TFTS) fabricated on an insulated substrate. It also relates to a semiconductor device having thin film transistors. Particularly, the present invention relates to an active matrix type semiconductor display. It also relates to a driving circuit of an active matrix semiconductor display.
2. Description of the Related Art
Techniques for fabricating thin film transistors (TFTS) by forming a semiconductor thin film on an inexpensive glass substrate have recently been under rapid development. The reason is increasing demands for active matrix liquid crystal displays (liquid crystal panels).
On an active matrix liquid crystal panel, a pixel TFT is provided in each of pixel regions in a large quantity in the range from several hundreds of thousands to several millions provided in the form of a matrix as shown in FIG. 33 (such a circuit is referred to as “active matrix circuit”). A switching element is provided for each pixel, and pixel information is controlled by turning the switching elements on and off. Liquid crystal is of used as a display medium of such a display. Particularly, elements having three terminals, i.e., thin film transistors (TFTs) having a gate, a source and a drain are used as the switching elements.
An active matrix circuit comprises thin film transistors made of amorphous silicon or polysilicon formed on a glass substrate.
Active matrix liquid crystal displays have recently been provided in which a quartz substrate is used and thin film transistors are formed by a polycrystalline silicon film. In this case, peripheral driving circuits for driving pixel TFTs can be also formed on the same substrate on which the active matrix circuit is formed.
A technique is also known which utilize processes such as laser annealing to fabricate thin film transistors on a glass substrate using a polycrystalline silicon film. The use of this technique allows the integration of an active matrix circuit and peripheral driving circuits on a glass substrate.
In the context of the present invention, a row of a matrix is associated with a thin film transistor whose gate electrode is connected to a scan line (gate line) provided in parallel with the row, and a column is associated with a thin film transistor whose source (or drain) electrode is connected to a signal line (source line) provided in parallel with the column. A circuit for driving scan lines is referred to as “scan line driving circuit”, and a circuit for driving signal lines is referred to as “scan line driving circuit”. A thin film transistor is referred to as “TFT”.
A back gate electrode is a pair of electrodes which are formed on a bottom side of a thin film transistor having a top gate type structure, i.e., formed toward the substrate and which are alternatively formed on a top side in case of a thin film transistor having a bottom gate type structure.
FIGS. 34A and 34B show a first example of conventional active matrix liquid crystal displays. In the active matrix liquid crystal display in this example, the thin film transistors made of amorphous silicon are used; the scan line driving circuit and signal line driving circuit are formed by integrated circuits 501 and 503 made of single crystal; and 502 represents an active matrix circuit which is mounted to the periphery of a glass substrate using tabs (FIG. 34A) or mounted using the COG (chip on glass) technique (FIG. 34B).
Such a liquid crystal display has had problems as described below. One problem has been the fact that problems with reliability can occur because the signal lines and scan lines of the active matrix are connected via tabs or bonding wires. For example, in the case VGA (video graphics array) type liquid crystal displays, there are 1920 signal lines and 480 scan lines, and those numbers of lines tend to increase as resolution is improved.
To fabricate a view finer used in a video camera or a projector utilizing liquid crystal, the display must be made compact, and a liquid crystal display with tabs has been disadvantageous from the viewpoint of space.
In order to solve those problems, active matrix liquid crystal displays have been developed in which the thin film transistors are formed from polysilicon. An example is shown in FIGS. 35A and 35B. As shown in FIG. 35A, a signal line driving circuit 401 and a scan line driving circuit 402 are formed on a glass substrate using polysilicon thin film transistors along with pixel thin film transistors to form an active matrix circuit. The formation of the polysilicon thin film transistors involves a high temperature polysilicon process in which processing is performed at 1000° C. or more on a quartz substrate to form the elements thereon or a low temperature polysilicon process in which processing is performed at 600° C. or less on a glass substrate to form the elements thereon.
The mobility of a polysilicon thin film transistor can be 30 cm2/Vsec. or more which enables an operation on a signal on the order of a few MHz, whereas the mobility of an amorphous thin film transistor is on the order of 0.5 cm2/Vsec.
There are digital type and analog type driving circuits for driving active matrix liquid crystal displays. The analog type is commonly used in a driving circuit utilizing polysilicon because the digital type includes a significantly greater number of circuit elements than those in the analog type. Circuit configurations utilizing a shift register are commonly used for scan line driving circuits and signal line driving circuits (See FIG. 35B). In FIG. 35B, DFF represents a delay flip-flop which operates in synchronism with a pulse applied to a clock terminal. When a start pulse HI is input to the DFF at the first stage, it outputs HI in synchronism with the clock, and the output is transferred to the second, third, - - - , N-th stages, which results in an operation of a shift register having N stages.
Recently, active matrix liquid crystal displays are widely used in notebook type personal computers. A personal computer requires a multiple tone liquid crystal display when a plurality of programs are activated simultaneously and when images from a digital camera are imported and processed therein.
Further, the recent spread of personal digital assistants, mobile computers, car navigation systems and the like has resulted in a need for compact active matrix liquid crystal displays with high fineness, resolution and image quality.
Compact projectors with high fineness, resolution and image quality utilizing an active matrix liquid crystal display are also attracting attention.
Obviously, TFTs forming a part of an active matrix liquid crystal display used for applications as described above must have high performance. When a TFT is discussed from the aspect of performance, reference is made to the mobility, threshold voltage and the like. Especially, there are considerably severe performance requirements on a threshold voltage. That is, a shift of a threshold voltage can result in malfunction of a driving circuit and pixel electrodes, which often makes it impossible to obtain a preferable image.
A conventional liquid crystal display as described above has had the following problems. It has been generally difficult to control the threshold voltage of a thin film transistor utilizing polysilicon when compared to a single crystal transistor, and this has sometimes put a transistor intended for the enhancement mode in the depression mode in which a current flows to the drain even when the gate-source voltage is 0. The reasons for this include the fact that it is less uniform compared to single crystal, the fact that a thermal oxide film can not be used as a gate oxide film in the case of low temperature polysilicon utilizing an inexpensive glass substrate because of the low heat-resisting properties of the glass substrate, and contamination with impurities from a glass substrate and fixed charges of an underlying film or the like formed to prevent contamination.
FIGS. 36A and 36B show gate voltage-drain current characteristics (Vg-Id curve) of a TFT. Let us assume that the thin film transistor which must have the characteristics shown in FIG. 36A actually has the characteristics shown in FIG. 36B because of a shift of the threshold. Then, no current flows in the inverter circuit shown in FIG. 37 when the input at the first stage is in a high state, whereas a current flows from the power supply to GND when the input is in a low state. At the next stage, a current conversely flows when the input is in the high state. When driving circuits of an active matrix liquid crystal display are provided as thin film transistors incorporated in a substrate, the stages at the signal side and scan side total at 2400 in the case of a VGA. This results in a high total current, although the current of each individual thin film is low. A significant problem has thus arisen from the viewpoint of reduction of power consumption of a display.
When the thin film transistors have an excessively great threshold voltage, the on current of the same becomes small, which has resulted in a problem in that the operating frequency of the driving circuits is decreased. Since load capacitance is driven by the on current of the thin film transistors, the operating frequency of the driving circuits is determined by the magnitude of the on current when the load capacitance and the power supply voltage are constant. Therefore, an excessively great threshold has resulted in a reduction of the operating frequency. Further, the trend toward more compact displays has resulted in a need for reduction of the size of driving circuits (reduction of the size of thin film transistors).
The present invention has been conceived taking the above-described problems with the prior art, and it is an object of the invention to control the threshold of thin film transistors by applying a voltage to the back gate electrodes, thereby reducing the power consumption of driving circuits and improving the operating frequency of the driving circuits. It is another object to reduce the size of thin film transistors by extracting a high current.